You are kidding, right? How can you wish for a rule like that? I mean.. the focal lenght so much influences your picture. Not just the dof, but the width of view and thus the amount of space/surroundings around your subject.
is shot at 35mm on 35mm equivalant. Showing exactly the amount of street that I wanted.
is shot at 85mm on 35mm equivalent, therefore not showing a lot more than just the girl. Photo 1 is shot in The Netherlands, the second one in NYC.. but you can't see that, because of the longer focal length.
If you would like to see that it's NYC, you'd need a different distance from your subject, or a different focal length.. as below.
So, I can not see why you would use a specific focal length just to control your sharpness. Use it to control your composition and the content of your image and use your aperture to control your sharpness.
And the way I work: just focus on the eye and about 1/3rd of the sharpness will be in front and 2/3rd will be behind the point of focus. But I DO want the eyes spot on sharp.. so I DO need to focus on the eyes.. the 2/3rd behind maybe kinda sharp, it's nog really sharp.
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I think you misunderstood what I was saying. For sure I was not saying use a specific focal length all the time. That would be, well, obvious I would think. But for specific focal lengths, what I was asking is how best do you try to keep the eyes, both eyes, in focus given a 45 degree to film plane. The further away the lens is from the model the more DOF you have at any one part of the body, including the eye sockets. As for ambient effect, I think what you describe is common knowledge even for serious amateur photographers, don't you think?
Which brings to lite a better question: How do you increase the DOF between the eyes, other than increasing aperture?
As I stated above, I "think" one method is using a 200mm lens as far back as you can for the frame you want, which increases DOF at the focal point. Of course if you are shooting a 3/4 body shot with a 50mm lens on a full frame camera at 10 feet, it's going to be quite easy to get the eyes sharp, as long as you aim at the model because your DOF is 2.94 ft total!
On the other hand, decrease that same scenario to 5 feet the DOF is .71 feet. In both cases you get sharp eyes by aiming at the closest eye.
I don't know how focal length and feet from subject would translate to how full the frame is, but just as an example using the same parameters as above, with these changes: 200mm at 20 feet gives you a DOF of .71 feet or about 8.5 inches. I would guess that at 20 feet using a 200mm lens compared to a 50mm at 10 feet would give you a head shot. If you wanted a head shot, without cropping, using the 50mm example you would need to be much closer. Let's say 3 feet which brings the DOF down to a 1/4 of a foot or 3 inches.
Hmmm. I'm going to look at my image again to see what I was doing. I'm beginning to think I was simply too close for the respective aperture and lens length I was using. I was in very tight places both times, once a hotel and the other a smaller front room with a grand piano in it. Three inches should be enough DOF to get both eyes sharp without a problem.
BTW I liked your images a lot.